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Type Parameters



Type parameters enable parameterized types.


& Name


A type parameter may occur at every syntactic position where a type is required and turns an ordinary type into a parameterized type. Parameterized types are used to define polymorphic functions and data types, i.e., functions and data types that are applicable for more than one type. Type parameters are bound to an actual type when the function or data type is applied and further uses of the type parameter are consistently replaced by the actual type.

The following syntactic positions are binding occurrences for type parameters:

  • Type parameters in the type declaration of a function are bound to the types of the actual parameters in the call of that function. Type parameters that occur in the body of the function are replaced by the corresponding actual types.

  • The left-hand side of an alias. The type parameters are bound when the alias is used and occurrences of type parameters in the right hand side are replaced by corresponding actual types.

  • The alternatives of a data type. Binding and replacement is identical to that of function declarations.

All other occurrences of type parameters are using occurrences. The following rules apply:

  • When the same type parameter is used at different binding occurrences it should be bound to the same actual type.

  • For every using occurrence of a type parameter there should be a binding occurrence of a type parameter with the same name.


Let's consider a small example of the use of function parameters in a function declaration, see Function for more details on function declarations. The following function swap returns a tuple in which its arguments are swapped and can be applied to arbitrary values in a type safe manner:

rascal>tuple[&B, &A] swap(&A a, &B b) { return <b, a>; }
tuple[&B,&A] (&A, &B): function(|prompt:///|(0,49,<1,0>,<1,49>))
tuple[int,int]: <2,1>
rascal>swap("abc", 3);
tuple[int,str]: <3,"abc">

Observe that the type parameters that are used in the return type should be defined in the declarations of the formal parameter of the function.

An Alias declaration may also be parameterized. So we can generalize graphs as follows:

alias Graph[&Node] = rel[&Node, &Node];
Graph[int] GI = {<1,2>, <3,4>, <4,1>};
Graph[str] GS = {<"a", "b">, <"c","d">, <"d", "a">};

The type parameters that are used in the type in the right part of the alias declaration should be defined in the left part of the alias definition.